In certain types of signal and control systems for railroad and mass and/or rapid transit operations, it is normal practice to regulate and control the acceleration and deceleration of a moving vehicle or train as it proceeds along its route of travel. Generally, speed command signals in the form of modulated or coded carrier wave forms are inductively coupled to the vehicle or train via a pick-up device. The picked-up modulated carrier signals are amplified, demodulated, shaped, limited, and decoded and filtered and are then employed to control to electrical state or condition of a speed decoding unit. In order to effectively control the positive and negative tractive apparatus, the actual speed of the moving vehicle or train must be compared with the picked-up and recovered speed command signal. In practice, an axle tachometer in the form of a frequency generator produces signals which are proportional to the actual speed of the moving vehicle or train. The recovered speed command signals and the actual speed signals are then compared to produce a differential signal to determine whether an accelerating or decelerating effort should be effected on the vehicle. In automatic train operation (ATO), the determination of whether positive or negative tractive effort should be applied to the vehicle is generally accomplished by digital logic circuits and networks. However, it will be appreciated that the differential speed signals are analog in nature, and thus an analog to digital (AN) conversion or interface circuit is necessary for coupling the analog speed sensing apparatus with the digital logic equipment. In the past, such interface circuits were very costly to construct and maintain due to the complexity and the excessive number of electrical and electronic components which were required to be used and assembled. In addition, the reliability of an electrical or electronic circuit or network may be greatly enhanced by simplifying and reducing the complexity of the circuit and by employing the minimal number of components. Thus, it is apparent that the optimum type of circuits or apparatus for car-carried equipment should be simple as possible in construction in order to minimize initial purchase and subsequent maintenance costs and also to maximize the efficiency, reliability and operability.